The present invention relates to controlling rotational speed of a motor installed in an electric power tool.
In one type of conventional electric power tools having a brushless DC (Direct Current) motor installed therein, in order to rotate a rotor of the motor, rotational positions of the rotor are detected based on electromotive force induced on each of a plurality of coils mounted in the motor. Then, rotational speed of the motor is calculated based on changes in detected rotational positions. The rotational speed of the motor is controlled by Proportional-Integral (PI) control such that the calculated rotational speed reaches target speed.
In such electric power tool, detection accuracy in rotational positions of a rotor is low, since the rotational positions of the rotor are not directly detected. Therefore, even when actual rotational speed of the motor is stabilized at target speed, calculated rotational speed may fluctuate. Due to the motor being controlled based on fluctuating calculated rotational speed, the rotation of the motor becomes unstable, thereby hunting or roaring noise of gears, on which driving force is applied from the motor, may be caused.
One of the conceivable solutions for such problem is disclosed in the Unexamined Japanese Patent Application Publication No. S61-094104. According to the control technique disclosed in this publication, a proportional gain is increased only when a deviation between calculated rotational speed and target speed is outside a predetermined range.
In an electric power tool to which such control technique is applied, although the rotational speed of the motor changes rapidly when the deviation is large, for example immediately after the motor of the electric power tool is started to be driven, change in the rotational speed of the motor becomes small when the deviation is small, for example when the rotational speed of the motor has reached to a vicinity of target speed.